The neonatal Fc receptor (FcRn) transports maternal IgG in ingested milk to the bloodstream of the newborn. IgG binds to FcRn on the apical side of intestinal epithelial cells at pH 6; FcRn/IgG complexes are transcytosed to the basolateral side of the cell where IgG is released at pH 7.4. The three-dimensional structures of FcRn and FcRn/Fc are known and suggest that FcRn dimerizes in response to IgG binding. The investigator hypothesizes that formation of an "oligomeric ribbon" of FcRn dimers on adjacent membranes bridged by IgG molecules is also required for FcRn function. This proposal aims at (i) understanding the roles of the FcRn dimer and oligomeric ribbon of FcRn dimers in IgG binding and transcytosis, (ii) dissecting the molecular basis for the pH dependence of the FcRn/IgG interaction, and (iii) understanding the significance of the closure of the FcRn counterpart of the MHC peptide-binding groove. To answer the first question, the binding affinities of site-directed mutants of FcRn which do not dimerize, and of Fc mutants that contact only one of the FcRn molecules in the FcRn dimer, will be measured by BIAcore. A hybrid Fc, in which a wild type Fc chain is paired with a mutant Fc chain that does not bind FcRn, will be used to determine if bridging of FcRn molecules by IgG is required for transcytosis. To answer the second question, Hill plot analysis of the pH-dependent affinity transitions of mutant complexes will be carried out to identify which residues mediate the transition. The crystal structure of FcRn at pH 8.0 will be determined and compared with that at pH 6.5 to address the possibility of pH-dependent conformational changes. To answer the third question, mutants of FcRn with an open, MHC-like peptide-binding groove will be constructed and characterized.